A quick correction there. Arm is part of the F18 project (llvm/flang) and we have helped upstream it and work on the driver, OpenMP portions etc. The core-compiler is developed by Nvidia Engineers (@pmk and team) with some contributions from others.
F18 has parsing support and a subset of checks for co-arrays. I believe there are plans for runtime but I am not sure of the timeline. Since there is some support for OpenMP parallel and worksharing constructs in F18 we are in a position to convert do-concurrent to these OpenMP constructs as one option, other options in future could be offloading to gpu or vectorisation or loop transformations.
This question was primarily in the context of Classic Flang. Classic Flang has sequential support for do-concurrent, but no support for co-arrays and no support for many Fortran 2008 intrinsics. I am trying to gauge what we are missing out while waiting for F18 to reach production.